Polymorphism arises when several [is the existence of] distinguishable phenotypes exist within a population of a species. Cepaea nemoralis, also known as the grove snail, is a highly variable land snail in the United Kingdom. Known to be one of the most polymorphic members of the European mollusc[s], the shell of this species has been subject to much investigation and was one of the earliest relating to polymorphism in genetics (Jones et al,1977). The observable phenotype of this species is an exact reflection of their genotype, which enables scientists a unique opportunity to compare interactions between genetics and ecology. Additionally, data for C. nemoralis has been collected for many years, resulting in a temporal time scale that the data from this investigation can be compared to. C. nemoralis can exhibit different shell colours, including yellow, pink and brown. These shell colours are one of the snail’s visible morphs, with the others being band presence and number ranging from zero to five. In addition, the presence of a dark brown peristome lip around the aperture of the shell identifies the snails as adults; juveniles do not have this feature. The natural habitat of C. nemoralis include gardens, woodlands, grasslands and hedges. Their staple diet tends to be vegetable matter; mostly dead and decaying plants; for example, fallen apples and dead grasses. The snails are hermaphrodites and reproduce during the spring and summer months were conditions are suitable i.e. when the weather is moist and warm. Despite large amounts of research within this species, there are sizable disagreements concerning findings, with one scientific belief being that natural selection is maintaining the genetic variation of polymorphism. Conflicting arguments propose the only major factor controlling the variation are random events in nature. C. nemoralis was presented as a prime example of the importance of random processes in evolution for many years, before studies in the connection between allele frequencies and environment were conducted. The main rationalisation for these random processes was genetic drift, the result of random mutations and spread or loss of these new alleles. Several studies on predation, climate and habitats have introduced alternative explanations to varying allele frequencies and polymorphism. Prior to these studies the shell colour or banding pattern were not seen as important factors affecting the organism’s life span or fecundity. In addition to genetic drift, climatic and visual and frequency dependent selection have been suggested to affect polymorphism. For example, different frequencies of dark coloured and banded/light coloured and non-banded snails in woodland and open grassland can be explained by all of these mechanisms, depending on the interpretation of the data (remembering that genetic drift and selection are not mutually exclusive). Within a population of a species, the expression of specific alleles is dependent upon the degree to which polymorphism occurs in said species; often a population only exhibits one particular allele. Once a specific allele portrays [has, or attains] a frequency of 100% across the population the allele/gene is deemed to be fixed. The prevalence of an allele, whether advantageous or not, is dependent upon two contributing factors; natural selection and genetic drift [gene flow? selection?]. Looking primarily at genetic drift, when an allele without any particular beneficial or unfavourable characteristics becomes fixed , it is due to random fluctuations within the species gene pool. In addition to environmental circumstances, genetic drift may have contributed to any dissociation of colour or banding observed on snail shells. The experimental approach observed phenotypes of C. nemoralis and analysed the frequency of those specific genotypes in relation to their habitual [don't be over-ornate] altitude. Data was collected in low and high altitudes across five sites. Data was collected in shrub-vegetated areas on one hillside of Monks Risborough, Oxfordshire, England. Altitude was chosen because the environments were varied between the valley of the hill and the peak of the hill. These varying conditions include factors such as wind exposure, moisture, sun exposure and nutrient availability. At a constant altitude differences in the genotype could be identified in the phenotype. A number of different variables were observed including shell colour, number of bands, adult vs. juvenile and living or dead. Keeping all the factors constant, the focus was on shell colour and band number in the various sites at distinct altitudes. Jones et al (1977) found that the alleles for pink and brown shell colour are dominant over the alleles for yellow shell colour. On this basis data collection grouped pink and brown shell colours together and yellow shell colour as one group. The raw data was organised into the following categories: black with pink 0-3 stripes, black with pink 4-5 stripes, yellow 0-3 stripes, and yellow 4-5 stripes. These categories were based upon ease of counting stripes, as the difference between 3 or fewer stripes and 4 or more stripes was the most obvious. The number of counts for each of these four categories at each altitude was calculated. The data was tested for statistical significance using the chi-squared test to check the validity of the null hypothesis. Using this test, one can analyse the results to determine if any differences in the data are purely due to chance or if there is likely to be a distinction between categories.
Michelle Hauge*
Catie Daubner
Priya Rehal
Liisa Peltola
Harriet Speed
Grace Challis
Polymorphism arises when several [is the existence of] distinguishable phenotypes exist within a population of a species. Cepaea nemoralis, also known as the grove snail, is a highly variable land snail in the United Kingdom. Known to be one of the most polymorphic members of the European mollusc[s], the shell of this species has been subject to much investigation and was one of the earliest relating to polymorphism in genetics (Jones et al,1977). The observable phenotype of this species is an exact reflection of their genotype, which enables scientists a unique opportunity to compare interactions between genetics and ecology. Additionally, data for C. nemoralis has been collected for many years, resulting in a temporal time scale that the data from this investigation can be compared to.
C. nemoralis can exhibit different shell colours, including yellow, pink and brown. These shell colours are one of the snail’s visible morphs, with the others being band presence and number ranging from zero to five. In addition, the presence of a dark brown peristome lip around the aperture of the shell identifies the snails as adults; juveniles do not have this feature.
The natural habitat of C. nemoralis include gardens, woodlands, grasslands and hedges. Their staple diet tends to be vegetable matter; mostly dead and decaying plants; for example, fallen apples and dead grasses. The snails are hermaphrodites and reproduce during the spring and summer months were conditions are suitable i.e. when the weather is moist and warm.
Despite large amounts of research within this species, there are sizable disagreements concerning findings, with one scientific belief being that natural selection is maintaining the genetic variation of polymorphism. Conflicting arguments propose the only major factor controlling the variation are random events in nature.
C. nemoralis was presented as a prime example of the importance of random processes in evolution for many years, before studies in the connection between allele frequencies and environment were conducted. The main rationalisation for these random processes was genetic drift, the result of random mutations and spread or loss of these new alleles. Several studies on predation, climate and habitats have introduced alternative explanations to varying allele frequencies and polymorphism. Prior to these studies the shell colour or banding pattern were not seen as important factors affecting the organism’s life span or fecundity. In addition to genetic drift, climatic and visual and frequency dependent selection have been suggested to affect polymorphism. For example, different frequencies of dark coloured and banded/light coloured and non-banded snails in woodland and open grassland can be explained by all of these mechanisms, depending on the interpretation of the data (remembering that genetic drift and selection are not mutually exclusive).
Within a population of a species, the expression of specific alleles is dependent upon the degree to which polymorphism occurs in said species; often a population only exhibits one particular allele. Once a specific allele portrays [has, or attains] a frequency of 100% across the population the allele/gene is deemed to be fixed. The prevalence of an allele, whether advantageous or not, is dependent upon two contributing factors; natural selection and genetic drift [gene flow? selection?]. Looking primarily at genetic drift, when an allele without any particular beneficial or unfavourable characteristics becomes fixed , it is due to random fluctuations within the species gene pool. In addition to environmental circumstances, genetic drift may have contributed to any dissociation of colour or banding observed on snail shells.
The experimental approach observed phenotypes of C. nemoralis and analysed the frequency of those specific genotypes in relation to their habitual [don't be over-ornate] altitude. Data was collected in low and high altitudes across five sites. Data was collected in shrub-vegetated areas on one hillside of Monks Risborough, Oxfordshire, England. Altitude was chosen because the environments were varied between the valley of the hill and the peak of the hill. These varying conditions include factors such as wind exposure, moisture, sun exposure and nutrient availability. At a constant altitude differences in the genotype could be identified in the phenotype. A number of different variables were observed including shell colour, number of bands, adult vs. juvenile and living or dead. Keeping all the factors constant, the focus was on shell colour and band number in the various sites at distinct altitudes. Jones et al (1977) found that the alleles for pink and brown shell colour are dominant over the alleles for yellow shell colour. On this basis data collection grouped pink and brown shell colours together and yellow shell colour as one group.
The raw data was organised into the following categories: black with pink 0-3 stripes, black with pink 4-5 stripes, yellow 0-3 stripes, and yellow 4-5 stripes. These categories were based upon ease of counting stripes, as the difference between 3 or fewer stripes and 4 or more stripes was the most obvious. The number of counts for each of these four categories at each altitude was calculated. The data was tested for statistical significance using the chi-squared test to check the validity of the null hypothesis. Using this test, one can analyse the results to determine if any differences in the data are purely due to chance or if there is likely to be a distinction between categories.